GCC: Mitigation against unsafe data speculation (CVE-2017-5753)

The patches I posted earlier this year for mitigating againstCVE-2017-5753 (Spectre variant 1) attracted some useful feedback, fromwhich it became obvious that a rethink was needed. This mail, and thefollowing patches attempt to address that feedback and present a newapproach to mitigating against this form of attack surface.[…]